1. Field of the Invention
The present invention relates to beam scanning systems for electromagnetic radiation and, more particularly, to systems in which beam deflection for scanning is a function of controlled variations of the wavelength of the radiation.
2. Summary Of The Invention
A system in which the beam of radiation from a laser or an equivalent source can be controllably directed has particular application in beam scanners, deflectors, and positioning devices that are employed for communications, target illumination, reconnaissance line scanning, medicine, and the like.
My invention is based on the fact that a dispersion element, such as a diffraction grating, will deflect a beam of radiation incident upon it in accordance with the wavelength of the radiation. In accordance with my invention, therefore, the beam of radiation that is to be used for scanning and the like purposes is deviated controllably by a dispersion element by selectively varying the wavelength of the radiation. Several embodiments of my invention utilize a multiplicity of radiation sources, each of a discrete wavelength and having their outputs directed at a dispersion element, and activate the sources selectively such that the output beam from the dispersion element is deviated controllably in accordance with the wavelength of the selected source. Other embodiments of my invention utilize a single radiation source and produce a deflection of the output beam by varying the wavelength of that single source. The wavelength of the output radiation of the single source is varied preferably by suitable means such as a parametric converter.
In this invention, a component of the radiation beam is utilized to monitor the output beam. Energy from the zero order of the radiation from the first dispersion element is passed to a second dispersion element which diffracts, refracts, or otherwise deflects is in synchronism with the primary output beam in accordance with the wavelength of the radiation. This second deviated beam is tracked by radiation sensors and information derived therefrom is used for system control functions.
In accordance with a specific preferred embodiment of my invention, a laser or its equivalent having an energy output that is of a substantially constant wavelength is used to generate an output radiation beam. This beam of radiation is directed through an interactor device such as an electro-optical parametric converter in which the original input wavelength can be transformed to a specified output wavelength in response to electrical signals applied to the device. Output radiation from the interactor device is directed to a first dispersion element such as an optical diffraction grating whose output is an undeviated zero orders beam and a deviated first order beam containing a major portion of the output energy. This high-energy principal output beam can be utilized for scanning or other tasks. In accordance with the laws of optics, the deflection caused by the dispersion element with respect to the angle of incidence of the input radiation of the first order output beam is a function of the wavelength of the input radiation so that, as the wavelength changes, the direction of the first order beam also changes.
The interaction characteristics of the parametric converter to change the wavelength of the radiation are controlled by time-varying electrical signals so that the direction of the first order output beam emanating from the first dispersion element oscillates between certain angular limits and the oscillations coincide with the time variation of the control signals imposed on the converter. The oscillations of the output beam to effect a desired pattern and frequency suitable for scanning or other tasks thus are governed by the characteristics of the control signals. In some instances, a simple sine wave excitation will suffice; in other instances the control signal may assume the characteristics of a sawtooth wave to provide a linear sweep in one direction and a rapid return of the beam to the starting point.
A feedback loop operating off the undeviated on-axis zero order output beam of the first dispersion element is provided for regulating the control signals to the parametric converter. In the feedback circuit, a second dispersion element such as a prism or a second grating is interposed in the path of the zero order beam from the first dispersive element. When the first dispersion element is a diffraction grating, its zero order output will be undeflected; however, when the wavelength of the radiation is varied, the beam output of the prism comprises the second dispersion element will oscillate in a similar manner and degree as the first order beam output of the first dispersive element. The excursions of the zero order beam will thus be a measure of the scanning beam with which it is synchronized. An array of photosensitive devices, such as photodiodes or photocells, is positioned in the path of the oscillating zero order beam from the prism and the output signal from any individual cell that is activated when the oscillating zero order beam impinges on it is an instantaneous measure of the position of the beam with respect to the array. This output of the photosensitive devices is used to regulate the control signal to the parametric interactor such that means for limiting the excursion of the output scanning beam oscillations and for synchronizing other auxiliary equipment to the oscillations are provided.
The radiation source of the invention can be of any suitable type such as a laser which may be of the liquid, solid, or gaseous type having either a discrete or continuous output. It goes without saying, of course, that the laser has to have a power output sufficient to meet the requirements. If a crystal-type parametric interactor is employed, it is also necessary that the laser has an operational wavelength which is suitably close to the degenerate frequency of the crystal used.
The two dominant features of any beam deflection system which are indicative of the efficiency or merit of the system are the deflection angle and the resolution. The deflection angle .DELTA..theta. is the amount of angular deviation that the system can impart to a beam of radiation, measured from some reference line such as the axis of the input beam. Resolution, N, is the ratio of the deflection angle .DELTA..theta. to the width, .phi., of the radiation beam, i.e., N=.DELTA..theta./.phi.. Resolution, thus, is the number of distinguishable spots that the beam from the system can assume.
It will be appreciated that radiation beam deflection systems have particular value for scanning, or the directing of a beam of electromagnetic energy successively over the elements of a given region; for convenience of exposition, therefore, the beam deflection system will be termed a "scanner" and the deflection itself will be referred to as "scanning". It will also be understood that, although the radiation may be in the visible range of the electromagnetic spectrum, wavelengths other than in the visible region in some cases may be more desirable and can be employed. Likewise, it is recognized that a laser is a preferred source of radiation, thus, the radiation source will be referred to as a "laser" and its output as a laser beam. It will be obvious, of course, that this choice of terminology is not to be construed to impose a limitation on the scope of this invention.
In addition to deflection angle and resolution, other parameters to be considered in evaluating laser scanners are listed and defined here for completeness.
(1) The operational wavelength is the fixed wavelength at which a scanner operates when there is no variation in wavelength, or the operational range over which the beam varies when the scanner does. In the present scanner, large variations in wavelength are a necessary prescription.
(2) The dimension of a scanning system indicates whether the scanner is capable of generating discrete spot positions in a line or plane. Tandem operation of one-dimensional scanners gives additional dimensional flexibility.
(3) Transit time relates directly to the time rate of scanning and is the time required for a wave of velocity, v, to propagate across an aperture, D, of the light beam to be deflected: EQU t=D/.sub.v.sbsb.a
where, v.sub.a, is the wave velocity. For example, where D=1 cm in fused quartz, in which v.sub.a =5.97.times.10.sup.5 cm/sec, the transit time is 1.67.times.10.sup.-6 seconds. A widely used figure of merit for scanners is the product of scan rate and resolution.
(4) Linearity refers to the relationship between time and beam position, i.e., the deviation with time in deflection from a linear relationship. This parameter becomes important in sequential scanners, such as those used in television, where a limit of 2% is often cited as an acceptable limit.
(5) Drive power is a consideration in the comparison between various scanning approaches when unique requirements of high voltage-low impedance are raised.
(6) The efficiency of a system is not clearly definable because of the wide interpretation that can be given to it. While the portion of the input beam deflected to a prescribed angle may represent high efficiency, that particular factor may not be of great significance in the overall performance of the system. Because of the uncertainties in an adequate definition of this parameter, such will not be attempted.